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Versions: (draft-huang-behave-bih) 00 01 02 03 04 05 06 07 08 09 RFC 6535

Behave WG                                                       B. Huang
Internet-Draft                                                   H. Deng
Obsoletes: 3338, 2767                                       China Mobile
(if approved)                                              T. Savolainen
Intended status: Standards Track                                   Nokia
Expires: January 26, 2012                                  July 25, 2011


            Dual Stack Hosts Using "Bump-in-the-Host" (BIH)
                     draft-ietf-behave-v4v6-bih-05

Abstract

   Bump-In-the-Host (BIH) is a host-based IPv4 to IPv6 protocol
   translation mechanism that allows a class of IPv4-only applications
   that work through NATs to communicate with IPv6-only peers.  The host
   on which applications are running may be connected to IPv6-only or
   dual-stack access networks.  BIH hides IPv6 and makes the IPv4-only
   applications think they are talking with IPv4 peers by local
   synthesis of IPv4 addresses.  This draft obsoletes RFC 2767 and RFC
   3338.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on January 26, 2012.

Copyright Notice

   Copyright (c) 2011 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents



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   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
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   Without obtaining an adequate license from the person(s) controlling
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   outside the IETF Standards Process, and derivative works of it may
   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.


































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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  Acknowledgement of previous work . . . . . . . . . . . . .  5
   2.  Components of the Bump-in-the-Host . . . . . . . . . . . . . .  6
     2.1.  Function Mapper  . . . . . . . . . . . . . . . . . . . . .  7
     2.2.  Protocol translator  . . . . . . . . . . . . . . . . . . .  8
     2.3.  Extension Name Resolver  . . . . . . . . . . . . . . . . .  8
       2.3.1.  Special exclusion sets for A and AAAA records  . . . .  9
       2.3.2.  DNSSEC support . . . . . . . . . . . . . . . . . . . .  9
       2.3.3.  Reverse DNS lookup . . . . . . . . . . . . . . . . . . 10
       2.3.4.  DNS cache in network-layer ENR . . . . . . . . . . . . 10
     2.4.  Address Mapper . . . . . . . . . . . . . . . . . . . . . . 10
   3.  Behavior and Network Examples  . . . . . . . . . . . . . . . . 12
   4.  Considerations . . . . . . . . . . . . . . . . . . . . . . . . 16
     4.1.  Socket API Conversion  . . . . . . . . . . . . . . . . . . 16
     4.2.  Socket bindings  . . . . . . . . . . . . . . . . . . . . . 16
     4.3.  ICMP Message Handling  . . . . . . . . . . . . . . . . . . 16
     4.4.  IPv4 Address Pool and Mapping Table  . . . . . . . . . . . 16
     4.5.  Multi-interface  . . . . . . . . . . . . . . . . . . . . . 17
     4.6.  Multicast  . . . . . . . . . . . . . . . . . . . . . . . . 18
   5.  Considerations due ALG requirements  . . . . . . . . . . . . . 19
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 20
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 21
   8.  Changes since RFC2767 and RFC3338  . . . . . . . . . . . . . . 22
   9.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 23
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 24
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 24
     10.2. Informative References . . . . . . . . . . . . . . . . . . 24
   Appendix A.  API list intercepted by BIH . . . . . . . . . . . . . 26
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 28




















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1.  Introduction

   This document describes Bump-in-the-Host (BIH), a successor and
   combination of the Bump-in-the-Stack (BIS)[RFC2767] and Bump-in-the-
   API (BIA) [RFC3338] technologies, which enable IPv4-only legacy
   applications to communicate with IPv6-only servers by synthesizing
   IPv4 addresses from AAAA records.

   The supported class of applications includes those that use DNS for
   IP address resolution and that do not embed IP address literals in
   protocol payloads.  This essentially includes legacy client-server
   applications using the DNS that are agnostic to the IP address family
   used by the destination and that are able to do NAT traversal.  The
   synthetic IPv4 addresses shown to applications are taken from the
   RFC1918 private address pool in order to ensure that possible NAT
   traversal techniques will be initiated.

   IETF recommends using dual-stack or tunneling based solutions for
   IPv6 transition and specifically recommends against deployments
   utilizing double protocol translation.  Use of BIH together with a
   NAT64 is NOT RECOMMENDED [RFC6180].

   BIH includes two major implementation options: a protocol translator
   between the IPv4 and the IPv6 stacks of a host, or an API translator
   between the IPv4 socket API module and the TCP/IP module.
   Essentially, IPv4 is translated into IPv6 at the socket API layer or
   at the IP layer.

   When BIH is implemented at the socket API layer, the translator
   intercepts IPv4 socket API function calls and invokes corresponding
   IPv6 socket API function calls to communicate with IPv6 hosts.

   When BIH is implemented at the networking layer the IPv4 packets are
   intercepted and converted to IPv6 using the IP conversion mechanism
   defined in Stateless IP/ICMP Translation Algorithm (SIIT) [RFC6145].
   The protocol translation has the same benefits and drawbacks as SIIT.

   The location of the BIH refers essentially to the location of the
   protocol translation function.  The location of DNS synthesis is
   orthogonal to the location of protocol translation, and may or may
   not happen at the same level.

   BIH can be used whenever an IPv4-only application needs to
   communicate with an IPv6-only server, independently of the address
   families supported by the access network.  Hence the access network
   can be IPv6-only or dual-stack capable.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",



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   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   [RFC2119] .

   This document uses terms defined in [RFC2460] and [RFC4213].

1.1.  Acknowledgement of previous work

   This document is a direct update to and directly derivative from
   Kazuaki TSHUCHIYA, Hidemitsu HIGUCHI, and Yoshifumi ATARASHI's Bump-
   in-the-Stack [RFC2767] and from Seungyun Lee, Myung-Ki Shin, Yong-Jin
   Kim, Alain Durand, and Erik Nordmark's Bump-in-the-API [RFC3338],
   which similarly provide a dual stack host means to communicate with
   other IPv6 hosts using existing IPv4 applications.  Section 7 covers
   the changes since those documents.




































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2.  Components of the Bump-in-the-Host

   Figure 1 shows the architecture of a host in which BIH is implemented
   as a socket API layer translator, i.e., as a "Bump-in-the-API".


                  +----------------------------------------------+
                  | +------------------------------------------+ |
                  | |                                          | |
                  | |            IPv4 applications             | |
                  | |                                          | |
                  | +------------------------------------------+ |
                  | +------------------------------------------+ |
                  | |           Socket API (IPv4, IPv6)        | |
                  | +------------------------------------------+ |
                  | +-[ API translator]------------------------+ |
                  | | +-----------+ +---------+ +------------+ | |
                  | | | Ext. Name | | Address | | Function   | | |
                  | | | Resolver  | | Mapper  | | Mapper     | | |
                  | | +-----------+ +---------+ +------------+ | |
                  | +------------------------------------------+ |
                  | +--------------------+ +-------------------+ |
                  | |                    | |                   | |
                  | |    TCP(UDP)/IPv4   | |   TCP(UDP)/IPv6   | |
                  | |                    | |                   | |
                  | +--------------------+ +-------------------+ |
                  +----------------------------------------------+

        Figure 1: Architecture of a dual stack host using protocol
                        translation at socket layer

   Figure 2 shows the architecture of a host in which BIH is implemented
   as a network layer translator, i.e., a "Bump-in-the-Stack".


















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      +------------------------------------------------------------+
      |  +------------------------------------------+              |
      |  |    IPv4 applications                     |              |
      |  |    Host's main DNS resolver              |              |
      |  +------------------------------------------+              |
      |  +------------------------------------------+              |
      |  |    TCP/UDP                               |              |
      |  +------------------------------------------+              |
      |  +------------------------------------------+ +---------+  |
      |  |    IPv4                                  | |         |  |
      |  +------------------------------------------+ | Address |  |
      |  +------------------+ +---------------------+ | Mapper  |  |
      |  |    Protocol      | |   Extension Name    | |         |  |
      |  |    Translator    | |   Resolver          | |         |  |
      |  +------------------+ +---------------------+ |         |  |
      |  +------------------------------------------+ |         |  |
      |  |    IPv4 / IPv6                           | |         |  |
      |  +------------------------------------------+ +---------+  |
      +------------------------------------------------------------+

        Figure 2: Architecture of a dual-stack host using protocol
                     translation at the network layer

   Dual stack hosts defined in RFC 4213 [RFC4213] need applications,
   TCP/IP modules and addresses for both IPv4 and IPv6.  The proposed
   hosts in this document have an API or network-layer translator to
   allow existing IPv4 applications to communicate with IPv6-only peers.
   The BIH architecture consists of an Extension Name Resolver, an
   Address Mapper, and depending on implementation either a Function
   Mapper or a Protocol Translator.  It is worth noting that the
   Extension Name Resolver's placement is orthogonal to the placement of
   protocol translation.  For example, the Extension Name Resolver may
   reside in the socket API while protocol translation takes place at
   the networking layer.

2.1.  Function Mapper

   The function mapper translates an IPv4 socket API function into an
   IPv6 socket API function.

   When detecting IPv4 socket API function calls from IPv4 applications,
   the function mapper intercepts the function calls and invokes IPv6
   socket API functions that correspond to the IPv4 socket API
   functions.

   The function mapper MUST NOT perform function mapping when the
   application is initiating communications to the address range used by
   local synthesis and the address mapping table does not have an entry



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   mathching the address.

   See Appendix B for a list of functions that MUST be intercepted by
   the function mapper.

2.2.  Protocol translator

   The protocol translator translates IPv4 into IPv6 and vice versa
   using the IP conversion mechanism defined in SIIT [RFC6145].  To
   avoid unnecessary fragmentation, the host's IPv4 module should be
   configured with a small enough MTU (IPv6 link MTU - 20 bytes).

   Protocol translation SHOULD NOT be performed for IPv4 packets sent to
   the IPv4 address range used by local synthesis and for which a
   mapping table entry does not exist.  The implementation SHOULD
   attempt to route such packets via IPv4 interfaces instead.

2.3.  Extension Name Resolver

   The Extension Name Resolver (ENR) returns a proper answer in response
   to the IPv4 application's name resolution request.

   In the case of the socket API layer implementation option, when an
   IPv4 application tries to do a forward lookup to resolve names via
   the resolver library (e.g., gethostbyname()), BIH intercepts the
   function call and instead calls the IPv6 equivalent functions (e.g.,
   getaddrinfo()) that will resolve both A and AAAA records.  This
   implementation option is name resolution protocol agnostic, and hence
   supports techniques such as "hosts-file", NetBIOS, mDNS, and
   essentially anything else the underlying operating system uses.

   In the case of the network layer implementation option, the ENR
   intercepts the A query and creates an additional AAAA query with
   essentially the same content.  The ENR will then collect replies to
   both A and AAAA queries and, depending on results, either return an A
   reply unmodified or synthesize a new A reply.  The network layer
   implementation option will only be able to catch applications' name
   resolution requests that result in actual DNS queries, hence is more
   limited when compared to the socket API layer implementation option.
   Hence the socket API layer option is RECOMMENDED.

   In either implementation option, if there is a real IPv4 address
   available, the ENR SHOULD NOT synthesize IPv4 addresses.  By default
   an ENR implementation MUST NOT synthesize IPv4 addresses when real
   IPv4 addresses exist.

   If only IPv6 addresses are available for the queried name, the ENR
   asks the address mapper to assign a local IPv4 address corresponding



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   to each IPv6 address.

   In the case of the API layer implementation option, the ENR will
   simply make the API (e.g. gethostbyname) return the synthetic
   address.  In the case of the network-layer implementation option, the
   ENR synthesizes an A record for the assigned IPv4 address, and
   delivers it up the stack.  If the response contains a CNAME or a
   DNAME record, then the CNAME or DNAME chain is followed until the
   first terminating A or AAAA record is reached.

   Application    | Network               | ENR behavior
     query        | response              |
   ---------------+-----------------------+----------------------------
 IPv4 address(es) | IPv4 address(es)      | return real IPv4 address(es)
 IPv4 address(es) | IPv6 address(es)      |  synthesize IPv4 address(es)
 IPv4 address(es) | IPv4/IPv6 address(es) | return real IPv4 address(es)

                    Figure 3: ENR behavior illustration

2.3.1.  Special exclusion sets for A and AAAA records

   An ENR implementation MAY by default exclude certain IPv4 and IPv6
   addresses seen on received A and AAAA records.  The addresses to be
   excluded by default SHOULD include martian addresses such as those
   that should not appear in the DNS or on the wire.  Additional
   addresses MAY be excluded based on possibly configurable local
   policies.

2.3.2.  DNSSEC support

   When the ENR is implemented at the network layer, the A record
   synthesis can cause essentially the same issues as are described in
   [RFC6147] section 3.  To avoid unwanted discarding of synthetic A
   records on the host's main resolver, the host's main resolver MUST
   send DNS questions with the CD "Checking Disabled" bit cleared.  The
   ENR can support DNSSEC as any resolver on a host.

   When the ENR is implemented at the socket API level, there are no
   problems with DNSSEC, as the ENR itself uses socket APIs for DNS
   resolution.

   DNSSEC can also be supported by configuring the (stub) resolver on a
   host to trust validations done by the ENR located at network layer or
   alternatively the validating resolver can implement the ENR itself.

   In order to properly support DNSSEC, the ENR SHOULD be implemented at
   the socket API level.  If the socket API level implementation is not
   possible, DNSSEC support SHOULD be provided by other means.



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2.3.3.  Reverse DNS lookup

   When an application requests a reverse lookup for an IPv4 address,
   the ENR MUST check whether the queried IPv4 address can be found in
   the Address Mapper's mapping table and is a local IPv4 address.  If
   an entry is found and the queried address is locally generated, the
   ENR MUST initiate a corresponding reverse lookup for the real IPv6
   address.  In the case where the application requested a reverse
   lookup for an address not part of the local IPv4 address pool, e.g.,
   a global address, the request MUST be passed on unmodified.

   For example, when an application requests a reverse lookup for a
   synthesized locally valid IPv4 address, the ENR needs to intercept
   that query.  The ENR asks the address mapper for the IPv6 address
   that corresponds to the IPv4 address.  The ENR shall perform a
   reverse lookup procedure for the destination's IPv6 address and
   return the name received as a response to the application that
   initiated the IPv4 query.

2.3.4.  DNS cache in network-layer ENR

   When BIH shuts down, e.g., due to an IPv4 interface becoming
   available, BIH MUST flush the node's DNS cache of possible locally
   generated entries.  This cache may be in the network-layer ENR
   itself, but also possibly host's caching stub resolver.

2.4.  Address Mapper

   The address mapper maintains a local IPv4 address pool.  The pool
   consists of private IPv4 addresses per section 4.3.  Also, the
   address mapper maintains a table consisting of pairs of locally
   selected IPv4 addresses and destinations' IPv6 addresses.

   When the extension name resolver, translator, or the function mapper
   requests the address mapper to assign an IPv4 address corresponding
   to an IPv6 address, the address mapper selects and returns an IPv4
   address out of the local pool, and registers a new entry into the
   table.  The registration occurs in the following 3 cases:

   (1) When the extension name resolver gets only IPv6 addresses for the
   target host name and there is no existing mapping entry for the IPv6
   addresses.  One or more local IPv4 addresses will be returned to the
   application and mappings for local IPv4 addresses to real IPv6
   addresses are created.

   (2) When the extension name resolver gets both IPv4 and IPv6
   addresses, but the IPv4 addresses contain only excluded IPv4
   addresses (e.g., 127.0.0.1).  The behavior will follow case (1).



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   (3) When the function mapper gets a socket API function call
   triggered by a received IPv6 packet and there is no existing mapping
   entry for the IPv6 source address (for example, the client sent a UDP
   request to an anycast address but a response was received from a
   unicast address).

   Other possible combinations are outside of BIH and BIH is not
   involved in those.











































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3.  Behavior and Network Examples

   Figure 4 illustrates a very basic network scenario.  An IPv4-only
   application is running on a host attached to the IPv6-only Internet
   and is talking to an IPv6-only server.  Communication is made
   possible by Bump-In-the-Host.

     +----+                                   +-------------+
     | H1 |----------- IPv6 Internet -------- | IPv6 server |
     +----+                                   +-------------+
     v4 only
     application

                       Figure 4: Network Scenario #1

   Figure 5 illustrates a possible network scenario where an IPv4-only
   application is running on a host attached to a dual-stack network,
   but the destination server is running on a private site that is
   numbered with public IPv6 addresses and private IPv4 addresses
   without port forwarding set up on the NAT44.  The only means to
   contact the server is to use IPv6.

     +----------------------+  +------------------------------+
     | Dual Stack Internet  |  | IPv4 Private site (Net 10)   |
     |                      |  | IPv6 routed site             |
     |                   +---------+             +----------+ |
     |                 +-|  NAT44  |-------------+          | |
     |  +----+         | +---------+             |          | |
     |  | H1 |---------+    |  |                 |  Server  | |
     |  +----+         | +-----------+           |          | |
     | v4 only         +-|IPv6 Router|-----------+          | |
     | application       +-----------+           +----------+ |
     |                      |  |                  Dual Stack  |
     |                      |  |                    10.1.1.1  |
     |                      |  |                 2001:DB8::1  |
     +----------------------+  +------------------------------+

                       Figure 5: Network Scenario #2

   Illustrations of host behavior in both implementation options are
   given here.  Figure 6 illustrates a setup where BIH (including the
   ENR) is implemented at the sockets API layer, and Figure 7
   illustrates a setup where BIH (including the ENR) is implemented at
   the network layer.







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 "dual stack"                                                "host6"
 IPv4    Socket |     [ API Translator ]    | TCP(UDP)/IP          Name
 appli-  API    | ENR      Address  Function| (v6/v4)             Server
 cation         |          Mapper   Mapper  |
  |        |        |        |        |         |              |       |
 <<Resolve IPv4 addresses for "host6".>>        |              |       |
  |        |        |        |        |         |              |       |
  |------->|------->|  Query IPv4 addresses for host6.         |       |
  |        |        |        |        |         |              |       |
  |        |        |------------------------------------------------->|
  |        |        |  Query 'A' and 'AAAA' records for host6          |
  |        |        |        |        |         |              |       |
  |        |        |<-------------------------------------------------|
  |        |        |  Reply with the 'AAAA' record.           |       |
  |        |        |        |        |         |              |
  |        |        |<<The 'AAAA' record is resolved.>>        |
  |        |        |        |        |         |              |
  |        |        |+++++++>|  Request one IPv4 address       |
  |        |        |        |  corresponding to the IPv6 address.
  |        |        |        |        |         |              |
  |        |        |        |<<Assign one IPv4 address.>>     |
  |        |        |        |        |         |              |
  |        |        |<+++++++|  Reply with the IPv4 address.   |
  |        |        |        |        |         |              |
  |<-------|<-------| Reply with the IPv4 address              |
  |        |        |        |        |         |              |
  |        |        |        |        |         |              |
 <<Call IPv4 Socket API function >>   |         |              |
  |        |        |        |        |         |              |
  |=======>|=========================>|An IPv4 Socket API function call
  |        |        |        |        |         |              |
  |        |        |        |<+++++++|  Request IPv6 addresses|
  |        |        |        |        |  corresponding to the  |
  |        |        |        |        |  IPv4 addresses.       |
  |        |        |        |        |         |              |
  |        |        |        |+++++++>| Reply with the IPv6 addresses.
  |        |        |        |        |         |              |
  |        |        |        |        |<<Translate IPv4 into IPv6.>>
  |        |        |        |        |         |              |
  |  An IPv6 Socket API function call.|=======================>|
  |        |        |        |        |         |              |
  |        |        |        |        |<<IPv6 data received    |
  |        |        |        |        |  from network.>>       |
  |        |        |        |        |         |              |
  |  An IPv6 Socket API function call.|<=======================|
  |        |        |        |        |         |              |
  |        |        |        |        |<<Translate IPv6 into IPv4.>>
  |        |        |        |        |         |              |



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  |        |        |        |<+++++++|  Request IPv4 addresses|
  |        |        |        |        |  corresponding to the  |
  |        |        |        |        |  IPv6 addresses.       |
  |        |        |        |        |         |              |
  |        |        |        |+++++++>| Reply with the IPv4 addresses.
  |        |        |        |        |         |              |
  |<=======|<=========================|  An IPv4 Socket function call.
  |        |        |        |        |         |              |

                 Figure 6: Example of BIH as API addition


      "dual stack"                                         "host6"
   IPv4 stub  TCP/    ENR     address  translator  IPv6
   app  res.  IPv4            mapper
     |   |    |       |         |       |           |         |
   <<Resolve an IPv4 address for "host6".>>         |         |
     |-->|    |       |         |       |           |         |
     |   |----------->|  Query 'A' records for "host6".       |  Name
     |   |    |       |         |       |           |         |  Server
     |   |    |       |------------------------------------------->|
     |   |    |       |  Query 'A' and 'AAAA'  records for "host6"
     |   |    |       |         |       |           |         |    |
     |   |    |       |<-------------------------------------------|
     |   |    |       |  Reply only with 'AAAA' record.       |
     |   |    |       |         |       |           |         |
     |   |    |       |<<Only 'AAAA' record is resolved.>>    |
     |   |    |       |         |       |           |         |
     |   |    |       |-------->|  Request one IPv4 address   |
     |   |    |       |         |  corresponding to each IPv6 address.
     |   |    |       |         |       |           |         |
     |   |    |       |         |<<Assign IPv4 addresses.>>   |
     |   |    |       |         |       |           |         |
     |   |    |       |<--------|  Reply with the IPv4 address.
     |   |    |       |         |       |           |         |
     |   |    |       |<<Create 'A' record for the IPv4 address.>>
     |   |    |       |         |       |           |         |
     |   |<-----------|  Reply with the 'A' record. |         |
     |   |    |       |         |       |           |         |
     |<--|<<Reply with the IPv4 address |           |         |
     |   |    |       |         |       |           |         |
     <<Send an IPv4 packet to "host6".>>|           |         |
     |   |    |       |         |       |           |         |
     |=======>|========================>|  An IPv4 packet.    |
     |   |    |       |         |       |           |         |
     |   |    |       |         |<++++++|  Request IPv6 addresses
     |   |    |       |         |       |  corresponding to the IPv4
     |   |    |       |         |       |  addresses.         |



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     |   |    |       |         |       |           |         |
     |   |    |       |         |++++++>|  Reply with the IPv6|
     |   |    |       |         |       |  addresses.         |
     |   |    |       |         |       |           |         |
     |   |    |       |         |       |<<Translate IPv4 into IPv6.>>
     |   |    |       |         |       |           |         |
     |   |    |       |An IPv6 packet.  |==========>|========>|
     |   |    |       |         |       |           |         |
     |   |    |       |         |   <<Reply with an IPv6 packet to.>>
     |   |    |       |         |       |           |         |
     |   |    |       |An IPv6 packet.  |<==========|<========|
     |   |    |       |         |       |           |         |
     |   |    |       |         |       |<<Translate IPv6 into IPv4.>>
     |   |    |       |         |       |           |         |
     |   |    |       |         |<++++++|Request IPv4 addresses
     |   |    |       |         |       |corresponding to the |
     |   |    |       |         |       |  IPv6 addresses.    |
     |   |    |       |         |       |           |         |
     |   |    |       |         |++++++>| Reply with the IPv4 addresses.
     |   |    |       |         |       |           |         |
     |<=======|=========================|  An IPv4 packet.    |
     |   |    |       |         |       |           |         |


               Figure 7: Example of BIH at the network layer


























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4.  Considerations

4.1.  Socket API Conversion

   IPv4 socket API functions are translated into IPv6 socket API
   functions that are semantically as identical as possible and vice
   versa.  See Appendix B for the API list intercepted by BIH.  However,
   some IPv4 socket API functions are not fully compatible with IPv6
   since IPv4 supports features that are not present in IPv6, such as
   SO_BROADCAST.

4.2.  Socket bindings

   BIH SHOULD select a source address for a socket from the recommended
   source address pool if a socket used for communications has not been
   explicitly bound to any IPv4 address.

   The binding of an explicitly bound socket MUST NOT be changed by the
   BIH.

4.3.  ICMP Message Handling

   When an application needs ICMP messages values (e.g., Type, Code,
   etc.) sent from the network layer, ICMPv4 message values MAY be
   translated into ICMPv6 message values based on SIIT [RFC6145], and
   vice versa.

4.4.  IPv4 Address Pool and Mapping Table

   The address pool consists of the private IPv4 addresses as per
   [RFC1918].  This pool can be implemented at different granularities
   in the node, e.g., a single pool per node, or at some finer
   granularity such as per-user or per-process.  In the case of a large
   number of IPv4 applications communicating with a large number of IPv6
   servers, the available address space may be exhausted if the
   granularity is not fine enough.  This should be a rare event and
   chances will decrease as IPv6 support increases.  The applications
   may use IPv4 addresses they learn for a much longer period than DNS
   time-to-live indicates.  Therefore, the mapping table entries should
   be kept active for a long period of time.  For example, a web browser
   may initiate one DNS query and then create multiple TCP sessions over
   time to the address it learns.  When address mapping table clean-up
   is required the BIH may utilize techniques used by network address
   translators, such as described in [RFC2663], [RFC5382], and
   [RFC5508].

   The RFC1918 address space was chosen because generally legacy
   applications understand it as a private address space.  A new



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   dedicated address space would run a risk of not being understood by
   applications as private. 127/8 and 169.254/16 are rejected due to
   possible assumptions applications may make when seeing those.

   The RFC1918 addresses used by the BIH have a risk of conflicting with
   addresses used in the host's possible IPv4 interfaces and
   corresponding local networks.  The conflicts can be mitigated, but
   not fully avoided, by using less commonly used portions of the
   RFC1918 address space.  Addresses from 172.16/12 are thought to be
   less likely to be in conflict than addresses from 10/8 or 192.168/16
   spaces.  A source address can usually be selected in a non-
   conflicting manner, but a small possibility exists for synthesized
   destination addresses being in conflict with real addresses used in
   local IPv4 networks.

   The RECOMMENDED IPv4 addresses are following:

   Primary source addresses: 172.21.112.0/30.  Source addresses have to
   be allocated because applications use getsockname() calls and in the
   network layer mode an IP address of the IPv4 interface has to be
   shown (e.g., by 'ifconfig').  More than one address is allocated to
   allow implementation flexibility, e.g., for cases where a host has
   multiple IPv6 interfaces.  The source addresses are from different
   subnets than destination addresses to ensure applications would not
   make on-link assumptions and would instead enable NAT traversal
   functions.

   Secondary source addresses: 10.170.224.0/20.  These addresses are
   recommended if a host has a conflict with primary source addresses.

   Primary destination addresses: 10.170.160.0/20.  The address mapper
   will select destination addresses primarily out of this pool.

   Secondary destination addresses: 172.21.80.0/20.  The address mapper
   will select destination addresses out of this pool if the node has a
   dual-stack connection conflicting with primary destination addresses.

4.5.  Multi-interface

   In the case of dual-stack destinations BIH MUST NOT do protocol
   translation from IPv4 to IPv6 when the host has any IPv4 interfaces,
   native or tunneled, available for use.

   It is possible that an IPv4 interface is activated during BIH
   operation, for example if a node moves to a coverage area of an IPv4-
   enabled network.  In such an event, BIH MUST stop initiating protocol
   translation sessions for new connections and BIH MAY disconnect
   active sessions.  The choice of disconnection is left for



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   implementations and it may depend on whether IPv4 address conflict
   occurs between addresses used by BIH and addresses used by the new
   IPv4 interface.

4.6.  Multicast

   Protocol translation for multicast is not supported.












































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5.  Considerations due ALG requirements

   No ALG functionality is specified herein as ALG design is generally
   not encouraged for host-based translation and as BIH is intended for
   applications that do not include IP addresses in protocol payloads.














































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6.  IANA Considerations

   There are no actions for IANA.
















































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7.  Security Considerations

   The security considerations of BIH mostly relies on that of
   [RFC6146].

   In the socket-layer implementation approach, the differences are due
   to the address translation occurring at the API and not in the
   network layer.  That is, since the mechanism uses the API translator
   at the socket API layer, hosts can utilize the security of the
   network layer (e.g., IPsec) when they communicate with IPv6 hosts
   using IPv4 applications via the mechanism.  As such, there is no need
   for DNS ALG as in NAT-PT, so there is no interference with DNSSEC
   either.

   In the network-layer implementation approach, IPv4-using IKE will not
   work.  This means IPv4-based IPsec/IKE using VPN solutions cannot
   work through BIH.  However, transport and application layer solutions
   such as TLS or SSL-VPN do work through BIH.

   The use of address pooling may open a denial-of-service attack
   vulnerability.  So BIH should employ the same sort of protection
   techniques as NAT64 [RFC6146] does.





























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8.  Changes since RFC2767 and RFC3338

   This document combines and obsoletes both [RFC2767] and [RFC3338].

   The changes in this document mainly reflect the following components:

      1.  Supporting IPv6-only network connections

      2.  The IPv4 address pool uses private address instead of reserved
      IPv4 addresses (0.0.0.1 - 0.0.0.255)

      3.  Extending ENR and address mapper to operate differently

      4.  Adding an alternative way to implement the ENR

      5.  Standards track instead of experimental/informational

      6.  Supporting reverse (PTR) queries

































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9.  Acknowledgments

   The author thanks the discussion from Gang Chen, Dapeng Liu, Bo Zhou,
   Hong Liu, Tao Sun, Zhen Cao, Feng Cao et al. in the development of
   this document.

   The efforts of Mohamed Boucadair, Dean Cheng, Lorenzo Colitti, Paco
   Cortes, Marnix Goossens, Ala Hamarsheh, Ed Jankiewizh, Suresh
   Krishnan, Julien Laganier, Yiu L. Lee, Jan M. Melen, Qibo Niu,
   Pierrick Seite, Christian Vogt, Magnus Westerlund, Dan Wing, and
   James Woodyatt in reviewing this document are gratefully
   acknowledged.

   Special acknowledgements go to Dave Thaler for his extensive review
   and support.

   The authors of RFC2767 acknowledged WIDE Project, Kazuhiko YAMAMOTO,
   Jun MURAI, Munechika SUMIKAWA, Ken WATANABE, and Takahisa MIYAMOTO.
   The authors of RFC3338 acknowledged implementation contributions by
   Wanjik Lee (wjlee@arang.miryang.ac.kr) and i2soft Corporation
   (www.i2soft.net).

   The authors of Bump-in-the-Wire (BIW) (draft-ietf-biw-00.txt, October
   2006), P. Moster, L. Chin, and D. Green, are acknowledged.  Some
   ideas and clarifications from BIW have been adapted to this document.


























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10.  References

10.1.  Normative References

   [RFC1918]  Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and
              E. Lear, "Address Allocation for Private Internets",
              BCP 5, RFC 1918, February 1996.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2460]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", RFC 2460, December 1998.

   [RFC4213]  Nordmark, E. and R. Gilligan, "Basic Transition Mechanisms
              for IPv6 Hosts and Routers", RFC 4213, October 2005.

   [RFC6145]  Li, X., Bao, C., and F. Baker, "IP/ICMP Translation
              Algorithm", RFC 6145, April 2011.

   [RFC6146]  Bagnulo, M., Matthews, P., and I. van Beijnum, "Stateful
              NAT64: Network Address and Protocol Translation from IPv6
              Clients to IPv4 Servers", RFC 6146, April 2011.

   [RFC6147]  Bagnulo, M., Sullivan, A., Matthews, P., and I. van
              Beijnum, "DNS64: DNS Extensions for Network Address
              Translation from IPv6 Clients to IPv4 Servers", RFC 6147,
              April 2011.

10.2.  Informative References

   [RFC2663]  Srisuresh, P. and M. Holdrege, "IP Network Address
              Translator (NAT) Terminology and Considerations",
              RFC 2663, August 1999.

   [RFC2767]  Tsuchiya, K., HIGUCHI, H., and Y. Atarashi, "Dual Stack
              Hosts using the "Bump-In-the-Stack" Technique (BIS)",
              RFC 2767, February 2000.

   [RFC3338]  Lee, S., Shin, M-K., Kim, Y-J., Nordmark, E., and A.
              Durand, "Dual Stack Hosts Using "Bump-in-the-API" (BIA)",
              RFC 3338, October 2002.

   [RFC3493]  Gilligan, R., Thomson, S., Bound, J., McCann, J., and W.
              Stevens, "Basic Socket Interface Extensions for IPv6",
              RFC 3493, February 2003.

   [RFC5382]  Guha, S., Biswas, K., Ford, B., Sivakumar, S., and P.



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              Srisuresh, "NAT Behavioral Requirements for TCP", BCP 142,
              RFC 5382, October 2008.

   [RFC5508]  Srisuresh, P., Ford, B., Sivakumar, S., and S. Guha, "NAT
              Behavioral Requirements for ICMP", BCP 148, RFC 5508,
              April 2009.

   [RFC6180]  Arkko, J. and F. Baker, "Guidelines for Using IPv6
              Transition Mechanisms during IPv6 Deployment", RFC 6180,
              May 2011.









































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Appendix A.  API list intercepted by BIH

   The following functions are the API list which SHOULD be intercepted
   by BIH module when implemented at socket layer.  Please note that
   this list may not be fully exhaustive.

   The functions that the application uses to pass addresses into the
   system are:

      bind()

      connect()

      sendmsg()

      sendto()

      gethostbyaddr()

      getnameinfo()

   The functions that return an address from the system to an
   application are:

      accept()

      recvfrom()

      recvmsg()

      getpeername()

      getsockname()

      gethostbyname()

      getaddrinfo()

   The functions that are related to socket options are:

      getsocketopt()

      setsocketopt()

   As well, raw sockets for IPv4 and IPv6 MAY be intercepted.

   Most of the socket functions require a pointer to the socket address
   structure as an argument.  Each IPv4 argument is mapped into



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   corresponding an IPv6 argument, and vice versa.

   According to [RFC3493], the following new IPv6 basic APIs and
   structures are required.

         IPv4                     new IPv6
         ------------------------------------------------
         AF_INET                  AF_INET6
         sockaddr_in              sockaddr_in6
         gethostbyname()          getaddrinfo()
         gethostbyaddr()          getnameinfo()
         inet_ntoa()/inet_addr()  inet_pton()/inet_ntop()
         INADDR_ANY               in6addr_any

                                 Figure 8

   BIH MAY intercept inet_ntoa() and inet_addr() and use the address
   mapper for those.  Doing that enables BIH to support literal IP
   addresses.  However, IPv4 address literals can only be used after a
   mapping entry between the IPv4 address and corresponding IPv6 address
   has been created.

   The gethostbyname() and getaddrinfo() calls return a list of
   addresses.  When the name resolver function invokes getaddrinfo() and
   getaddrinfo() returns multiple IP addresses, whether IPv4 or IPv6,
   they SHOULD all be represented in the addresses returned by
   gethostbyname().  Thus if getaddrinfo() returns multiple IPv6
   addresses, this implies that multiple address mappings will be
   created; one for each IPv6 address.






















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Authors' Addresses

   Bill Huang
   China Mobile
   53A,Xibianmennei Ave.,
   Xuanwu District,
   Beijing  100053
   China

   Email: bill.huang@chinamobile.com


   Hui Deng
   China Mobile
   53A,Xibianmennei Ave.,
   Xuanwu District,
   Beijing  100053
   China

   Email: denghui02@gmail.com


   Teemu Savolainen
   Nokia
   Hermiankatu 12 D
   FI-33720 TAMPERE
   Finland

   Email: teemu.savolainen@nokia.com






















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